Burning natural gas produces half as much CO2 as coal.
The US has about 500 coal fired power plants. If Obama shut them all down, and they were replaced by gas power plants, that would reduce the amount of CO2 generated by about 250 power plants.
The rest of the world has 1,200 new coal fired plants in the planning or building stages, meaning that our 250 plant reduction would be overwhelmed by the all new coal plants being built elsewhere.
So at massive cost we could accomplish almost nothing wrt CO2, while adding more of the super evil greenhouse gas CH4 to the atmosphere.